2025/7/28
Observation of the Largest Binary Black Hole Merger Ever: GW231123
On July 14, 2025, the LIGO-Virgo-KAGRA (LVK) collaboration announced the detection of GW231123, the most massive binary black hole merger ever observed via gravitational waves. This gravitational wave signal was captured by two detectors in the United States: LIGO Hanford and LIGO Livingston.
Key Features of GW231123
- Largest Black Holes Ever Observed: The two black holes that merged had estimated masses of approximately 100 and 140 times that of the Sun, respectively. The resulting black hole formed from the merger is estimated to have a mass of 225 solar masses—making it the most massive black hole ever detected through gravitational waves.
- Challenge to the Mass Gap: Notably, both pre-merger black holes likely fell within the so-called “mass gap” range of 60 to 130 solar masses. This range is considered difficult to reach through standard stellar evolution, suggesting new possibilities for black hole formation.
- High Spin Rates: It was also revealed that the pre-merger black holes had extremely high spin rates—between 80% and 90% of the theoretical maximum allowed for black holes.
Unraveling the Mystery of Black Hole Formation
One possible scenario for the formation of such massive and rapidly spinning black holes is hierarchical mergers, where smaller black holes repeatedly merge over time, gradually increasing in mass. This scenario naturally explains both the large mass and high spin observed in this event.
However, hierarchical mergers can lead to non-circular orbits and precession of the black holes’ spin axes. Unfortunately, the duration of the gravitational wave signal from GW231123 was very short, making it difficult to detect such effects.
The observation of GW231123 marks a groundbreaking step forward in our understanding of the universe. Continued advancements in gravitational wave astronomy are expected to further unravel the mysteries of the cosmos.
For more details, please refer to the summary provided by the LVK collaboration below.
Science Summary:
English: https://ligo.org/wp-content/uploads/2025/07/GW231123.pdf
Other languages: https://ligo.org/science-summaries/
LIGO Announcement Page: https://ligo.org/ligo-virgo-kagra-detect-most-massive-black-hole-merger-to-date/
https://ligo.org/detections/gw231123/
Paper (arXiv): https://arxiv.org/abs/2507.08219
LVK Collaboration Press Release (English): https://ligo.org/wp-content/uploads/2025/07/Press-release-for-GW231123-discovery-paper-final.pdf